Sofc system with selective co2 removal

ABSTRACT

A system and method in which a high temperature fuel cell stack exhaust stream is recycled back into the fuel inlet stream of the high temperature fuel cell stack. The recycled stream may be sent to a carbon dioxide separation device which separates carbon dioxide from the fuel exhaust stream. The carbon dioxide separation device may be a carbon dioxide trap, an electrochemical carbon dioxide separator, or a membrane separator. A water separator may be used in conjunction with the carbon dioxide separation device or used separately to continuously remove water from the recycled stream.

BACKGROUND

The present invention relates generally to the field of fuel cellsystems and more particularly to fuel cell systems integrated withcarbon dioxide removal components.

Fuel cells are electrochemical devices which can convert energy storedin fuels to electrical energy efficiencies. High temperature fuel cellsinclude solid oxide and molten carbonate fuel cells. These fuel cellsmay operate using hydrogen and/or hydrocarbon fuels. There are classesof fuel cells, such as the solid oxide regenerative fuel cells, thatalso allow reversed operation, such that oxidized fuel can be reducedback to unoxidized fuel using electrical energy as an input.

SUMMARY OF THE INVENTION

The embodiments of the invention provide a system and method in which ahigh temperature fuel cell stack exhaust stream is recycled back intothe fuel inlet stream of the high temperature fuel cell stack. Therecycled stream may be sent to a carbon dioxide separation device whichseparates carbon dioxide from the fuel exhaust stream. The carbondioxide separation device may be a carbon dioxide trap, anelectrochemical carbon dioxide separator, or a membrane separator. Theremoval of carbon dioxide from the recycled anode exhaust increases theefficiency of the high temperature fuel cell stack. In one aspect of theinvention, a water separator is used in conjunction with the carbondioxide separation device to continuously remove water from the recycledstream. The removal of water from the recycled anode exhaust streamincreases cell performance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic of a fuel cell system according to an embodimentof the present invention.

FIG. 1B is a schematic of a carbon dioxide separator of FIG. 1A.

FIG. 2 is a schematic of a carbon dioxide separator capable of use withthe embodiments of the present invention.

FIGS. 3-15 are schematics of fuel cell systems according to embodimentsof the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The embodiments of the invention illustrate how carbon dioxideseparation devices may be used together with a fuel cell system, such asa solid oxide fuel cell system. Additional embodiments illustrate howwater separation devices may be used together with a fuel cell system,such as a solid oxide fuel cell system. Additional embodimentsillustrate how carbon dioxide separation devices and water separationdevices may be used together with a fuel cell system, such as a solidoxide fuel cell system. It should be noted that other fuel cell systems,such as molten carbonate systems, may also be used.

FIG. 1 illustrates a fuel cell system 100 according to one embodiment ofthe present invention. Preferably, the system 100 is a high temperaturefuel cell stack system, such as a solid oxide fuel cell (SOFC) system.The system 100 may be a regenerative system such as a solid oxideregenerative fuel cell (SORFC) system which operates in both fuel cell(i.e., discharge or power generation) and electrolysis (i.e., charge)modes or it may be a non-regenerative system which only operates in thefuel cell mode.

The system 100 contains a high temperature fuel cell stack 106. Thestack may contain a plurality of SOFCs or SORFCs. The high temperaturefuel cell stack 106 is illustrated schematically to show one solid oxidefuel cell of the stack containing a ceramic electrolyte, such as yttriaor scandia stabilized zirconia, an anode electrode, such as anickel-stabilized zirconia cermet, and a cathode electrode, such aslanthanum strontium manganite. Each fuel cell contains an electrolyte,an anode electrode on one side of the electrolyte anode chamber, acathode electrode on the other side of the electrolyte in a cathodechamber, as well as other components, such as separatorplates/electrical contacts, fuel cell housing and insulation. In an SOFCoperating in the fuel cell mode, the oxidizer, such as air or oxygengas, enters the cathode chamber, while the fuel, such as hydrogen orhydro-carbon fuel, enters the anode chamber. Any suitable fuel celldesigns and component materials may be used. The system 100 furthercontains an anode tail gas oxidizer (ATO) reactor 116, a recirculationblower 122, and a canister carbon dioxide trap 126.

The system 100 operates as follows. The fuel inlet stream is providedinto the fuel cell stack 106 through fuel inlet conduit 102. The fuelmay comprise any suitable fuel, such as a hydrogen fuel or a hydrocarbonfuel, including but not limited to methane, natural gas which containsmethane with hydrogen and other gases, propane or other biogas, or amixture of a carbon fuel, such as carbon monoxide, oxygenated carboncontaining gas, such as methanol, or other carbon containing gas with ahydrogen containing gas, such as water vapor, hydrogen gas or othermixtures. For example, the mixture may comprise syngas derived from coalor natural gas reformation. The fuel inlet conduit 102 provides the fuelinlet stream to the anode side of the fuel cell stack 106.

Air or another oxygen containing gas is provided into the stack 106through an air inlet conduit 104. The air inlet conduit 104 provides airto the cathode side of the fuel cell stack 106.

Once the fuel and oxidant are provided into the fuel cell stack 106, thestack 106 is operated to generate electricity and a fuel exhaust stream.The fuel exhaust stream may contain hydrogen, water vapor, carbonmonoxide, carbon dioxide, some un-reacted hydrocarbon gas, such asmethane, and other reaction by-products and impurities.

The fuel exhaust stream (i.e., the stack anode exhaust stream) isprovided from the stack 106 via fuel exhaust conduit 110. The airexhaust stream (i.e., the stack cathode exhaust stream) is provided fromthe stack air exhaust outlet via air exhaust conduit 112. The fuelexhaust conduit 110 is configured to provide a portion of the fuelexhaust stream to the ATO reactor 116 via ATO input conduit 114 andrecycle a portion of the fuel exhaust stream via recycling conduit 120.The portion of fuel exhaust provided to the ATO reactor 116 and recycledvia recycling conduit 120 may vary. For example 10% of the fuel exhaustmay be provided to the ATO reactor 116 and 90% recycled. Alternatively,50% of the fuel exhaust may be provided to the ATO reactor 116, while50% is recycled. Additionally, 90% of the fuel exhaust or more may beprovided to the ATO reactor, while 10% or less is recycled. The amountof recycled fuel provided into conduit 120 is controlled by blower 122power or blowing speed. The fuel exhaust stream provided into conduits114 and 120 may contain the same composition or content of hydrogen,carbon monoxide, water, and carbon dioxide. Air exhaust conduit 112 isconfigured to provide the air exhaust stream to the ATO reactor 116.

The ATO reactor 116 receives the fuel exhaust stream and air exhauststream via ATO input conduit 114 and conduit 112, respectively. The ATOreactor uses the combined fuel exhaust stream and air exhaust stream tooxidize anode tail gas and output heated oxidized fuel (i.e., reactorexhaust) to ATO exhaust conduit 118.

A recirculation blower 122 is coupled to recycling conduit 120 toprovide the recycled fuel exhaust stream from recycling conduit 120 to acarbon dioxide canister trap 126 via recycling conduit 124. Therecirculation blower 122 may be computer or operator controlled and mayvary the amount and/or rate of the recycled fuel exhaust stream beingprovided to the carbon dioxide canister trap 126 and also the amountand/or rate of the carbon dioxide free or carbon dioxide depletedrecycled fuel exhaust stream being provided back to the stack 106. Assuch, the recirculation blower 122 may be used to increase or decreasethe overall recycling rate in system 100.

The carbon dioxide canister trap 126 may be any type carbon dioxidetrap, such as a consumable carbon dioxide trap. The carbon dioxidecanister trap 126 has no carbon dioxide conduit. Instead, the carbondioxide canister trap 126 is physically removed from the SOFC system byan operator when it fills up with carbon dioxide and is replaced with aclean trap. The carbon dioxide canister trap 126 may be placeddownstream of the recirculation blower 122 and may be used to extendhotbox life so long as the carbon dioxide canister trap 126 may beroutinely replaced. The carbon dioxide canister trap 126 removes carbondioxide from the recycled fuel exhaust stream. Preferably, the carbondioxide canister trap 126 removes substantially all the carbon dioxidefrom the recycled fuel exhaust stream. The carbon dioxide canister trap126 may remove less than 50%, or more than 50%, such as 50% to 60%, 60%to 70%, 70% to 80%, 80% to 90%, or 90% to 100%, such at about 98%, about99%, or about 99.5% of the carbon dioxide from the recycled fuel exhauststream. The carbon dioxide canister trap 126 may require daily changeout of its carbon dioxide gathering components, or other suitable changeout periodicity may be required. Bypass valve and conduit (not shown)may be provided to allow carbon dioxide canister trap 126 replacement ofwithout power generation interruption. Preferably the carbon dioxidecanister trap 126 is located outside of the hot box containing the fuelstack 106 for easy access for service personnel. Carbon dioxide canistertrap 126 may be located in system housing containing the hot box.

FIG. 1B illustrates a schematic of a carbon dioxide canister trap 126 ofFIG. 1A. The carbon dioxide canister trap 126 is shown in greater detailin FIG. 1B. The carbon dioxide canister trap 126 may be comprised of twocarbon dioxide canister traps 126A and 126B. A valve 125 may be providedto allow the diversion of the recycled fuel exhaust stream fromrecycling conduit 124 to one or both of carbon dioxide canister traps126A or 126B. Additionally the valve 125 may prevent the recycled fuelexhaust stream from flowing to one or both of carbon dioxide canistertraps 126A and 126B. Carbon dioxide canister traps 126A and 126B removecarbon dioxide from the recycled fuel exhaust stream. A valve 127 may beprovided to allow the isolation of carbon dioxide canister traps 126Aand 126B from recycling conduit 128. The operation of valve 125 andvalve 127 may allow a system operator to pass recycled fuel exhaust toone, both, or neither of the carbon dioxide canister traps 126A and 126Bat the same time. Valve 125 and valve 127 may be configured to isolateeither carbon dioxide canister trap 126A and 126B from system 100. Inother words, carbon dioxide canister trap 126B may be isolated andreplace while carbon dioxide trap 126A continues to function, and viceversa. This isolation may facilitate trap change out or othermaintenance or regulate the rate of carbon dioxide removal without powergeneration interruption.

As illustrated in both FIGS. 1A and 1B the purified recycled fuelexhaust stream, with a reduced amount of carbon dioxide, is providedback to the fuel inlet stream for the fuel stack 106 via recyclingconduit 128. The recycling of carbon dioxide depleted fuel exhaust intothe fuel inlet increases the performance of the fuel cell stack 106.

FIG. 2 illustrates an electrochemical carbon dioxide separator 226according to another embodiment of the present invention. Theelectrochemical carbon dioxide separator 226 is one type of carbondioxide separator which may be used with embodiments of the presentinvention. The electrochemical carbon dioxide separator 226 may be amolten carbonate fuel cell operated in electrolysis mode (i.e., withapplied potential).

The electrochemical carbon dioxide separator 226 may receive a recycledfuel exhaust stream input via recycling conduit 224. The recycled fuelexhaust stream may consist of hydrogen, carbon dioxide, water, andcarbon dioxide. The recycling conduit 224 may be coupled to the anode206 chamber of the electrochemical carbon dioxide separator 226. Air isprovided to the electrochemical carbon dioxide separator 226 via airinput conduit 202 and used to purge the electrochemical carbon dioxideseparator 226. Electricity is applied to the electrochemical carbondioxide separator 226 from a power supply 204 to operate electrochemicalcarbon dioxide separator in electrolyzer mode. In an embodiment, thepower supply 204 may comprise the fuel cell stack 106. The currentapplied transfers carbonate ions (CO₃ ⁻²) from the anode 206, throughthe electrolyte 208, to the cathode 210 according to the followingreaction:

Anode: 2H₂O→2H₂+O₂ O₂+2CO₂+2e⁻→CO₃ ⁻²

Cathode: CO₃ ^(%31 2)→O₂+2CO₂+2e⁻

The cathode 210 chamber is coupled to a carbon dioxide conduit 214 andcarbon dioxide extracted from the recycled fuel exhaust stream exits theelectrochemical carbon dioxide separator 226 via the carbon dioxideconduit 214.

The anode 206 chamber is further coupled to a purified recycled fuelexhaust stream conduit 212. Purified recycled fuel exhaust streamexiting the carbon dioxide separator anode 206 chamber via the purifiedanode exhaust conduit 212 contains less carbon dioxide than the recycledfuel exhaust stream that entered the carbon dioxide separator 226 viathe recycling conduit 224. As a percentage of overall composition, thepurified recycled fuel exhaust stream in the purified recycled fuelexhaust stream conduit 212 contains a greater percentage of hydrogenthan the recycled fuel exhaust stream entering the carbon dioxideseparator 206 via recycling conduit 224. Preferably, the electrochemicalcarbon dioxide separator 226 removes substantially all the carbondioxide from the recycled fuel exhaust stream. The electrochemicalcarbon dioxide separator 226 may remove less than 50%, or more than 50%,such as 50% to 60%, 60% to 70%, 70% to 80%, 80% to 90%, or 90% to 100%,such at about 98%, about 99%, or about 99.5% of the carbon dioxide fromthe recycled fuel exhaust stream.

FIG. 3 illustrates a system 300 according to an embodiment of theinvention. The system 300 is similar to system 100 illustrated in FIG. 1and contains a number of components in common. Those components whichare common to both systems 100 and 300 are numbered with the samenumbers in FIGS. 1 and 3 and will not be described further.

One difference between systems 100 and 300 is that system 300 mayutilize a carbon dioxide separator 326 as opposed to a carbon dioxidecanister trap 126. The carbon dioxide separator 326 may be any typecarbon dioxide separator, such as a carbon dioxide membrane separator oran electrochemical carbon dioxide separator as discussed in relation toFIG. 2 above. Another difference between systems 100 and 300 is thatsystem 300 may utilize ATO exhaust or SOFC cathode exhaust to sweep thecollection side of the carbon dioxide separator 326 to remove carbondioxide. An additional difference between systems 100 and 300 is thatsystem 300 may bias the carbon dioxide separator 326 collection side gaswith water.

Recycling conduit 124 may be coupled to the carbon dioxide separator326. The recycled fuel exhaust stream is input to the carbon dioxideseparator 326 via the recycling conduit 124, and carbon dioxide isremoved from the recycled fuel exhaust stream to produce a purified(e.g., carbon dioxide depleted) recycled fuel exhaust stream. Thepurified recycled fuel exhaust stream exiting the carbon dioxideseparator 326 contains less carbon dioxide than the recycled fuelexhaust stream that entered the carbon dioxide separator 326 via therecycling conduit 124. As a percentage of overall composition thepurified recycled fuel exhaust stream contains a greater percentage ofhydrogen than the recycled fuel exhaust stream entering the carbondioxide separator 326 via recycling conduit 124. Preferably, the carbondioxide separator 326 removes substantially all the carbon dioxide fromthe recycled fuel exhaust stream. The carbon dioxide separator 326 mayremove less than 50%, or more than 50%, such as 50% to 60%, 60% to 70%,70% to 80%, 80% to 90%, or 90% to 100%, such at about 98%, about 99%, orabout 99.5% of the carbon dioxide from the recycled fuel exhaust stream.

The carbon dioxide separator 326 is coupled to recycling conduit 334.The purified recycled fuel exhaust stream, with a reduced amount ofcarbon dioxide, is provided back to the fuel inlet stream by therecycling conduit 334. The recycling of reduced carbon dioxide fuelexhaust into the fuel inlet increases the performance of the fuel cellstack 106.

The efficiency of the carbon dioxide separator 326 at selecting forcarbon dioxide is increased by the biasing of the collection side of thecarbon dioxide separator 326 by adding water to the collection side ofthe carbon separator 326.

In one embodiment, the hot exhaust from the ATO reactor 116 is passedvia hot exhaust conduit 118 to a cathode recuperator heat exchanger 336where the ATO exhaust exchanges heat with the air inlet stream providedthrough air inlet conduit 104. The heat exchanger helps to raise thetemperature of the air in air inlet conduit 104 and reduces thetemperature of the ATO exhaust in conduit 118 such that it does notdamage the membrane humidifier 328.

In an alternative embodiment, all or a portion of the SOFC cathodeexhaust may be passed directly to the cathode recuperator heat exchanger336. A valve 349 may direct cathode exhaust from conduit 112 to conduit350. Valve 349 may alternatively be a splitter (not shown) configured todirect a portion of the cathode exhaust to conduit 350 and a portion ofthe cathode exhaust to the ATO reactor. Valve 351 may be configured todirect the cathode exhaust received from conduit 350 toward the cathoderecuperator heat exchanger 336 and prevent cathode exhaust from flowingto the ATO reactor 116. Additionally, valve 351 may be coupled to aconduit 352 to direct ATO exhaust and/or SOFC cathode exhaust out of thesystem 300 as exhaust. The utilization of valves 349 and 351 and conduit350 may allow either SOFC cathode exhaust or ATO exhaust, a mixture ofboth ATO exhaust and SOFC cathode exhaust, or neither ATO exhaust norSOFC cathode exhaust to pass to the cathode recuperator heat exchanger336.

From the heat exchanger 336, the ATO exhaust conduit 118 may be coupledto a membrane humidifier 328. Air is input to the membrane humidifier328 via conduit 118. Optionally, air may also be input to the membranehumidifier as via air conduit 340 coupled to the membrane humidifier328. Air conduit 340 may input air supplied by a blower, fan, orcompressor (not shown).

In operation, the membrane humidifier 328 humidifies an air or oxidizedfuel stream for input into the carbon dioxide separator 326. Themembrane humidifier 328 may comprise a polymeric membrane humidifier.

Water may be input to the membrane humidifier 328 via a water conduit342 as necessary. Water is also may be collected by the membranehumidifier 328 from the carbon dioxide conduit 332, which is coupledbetween the carbon dioxide separator 326 and the membrane humidifier332. The water permeates across the membrane from product side 328B tocollection side 328A of membrane humidifier 328. The water from theconduit 342 is mixed in the membrane humidifier 328 with the ATO exhaustfrom conduit 118 and the now humid air passes to humid air conduit 330.

Humid air conduit 330 is coupled to the carbon dioxide separator 326 andthe humid air or ATO exhaust is used to bias the separation of carbondioxide by the carbon dioxide separator 326. Where a traditional carbondioxide separator naturally selects for water in a reaction, thepresence of water on the collection side of the carbon dioxide separatorreduces the selection of water and increases the efficiency of thecarbon dioxide separator to select for carbon dioxide. In this mannerthe increased amount of water in the air entering the collection side ofthe carbon dioxide separator 326 biases the carbon dioxide separator 326to select for carbon dioxide from the recycled fuel exhaust stream.Preferably, the humid air or ATO exhaust contains a substantially equalamount of water as the recycled fuel exhaust stream. The humid air orATO exhaust may contain about 90%, 80%, 70%, 60%, 50%, 40%, 30%, 20%, or10% of the water contained in the recycled fuel exhaust stream. The term“about” provides a variation based on given processes variables, such asa variation of 10% or less, preferably 5% or less. The humid air or ATOexhaust may also contain more than 100% of the water contained in therecycled fuel exhaust stream, such as about 110%, 120%, 130%, 140%,150%, 160%, 170%, 180%, 190%, or 200%.

Thus conduit 330 inputs a humid mix into the collection side 326A andconduit 332 outputs a carbon dioxide and humid mix from the collectionside 326A of carbon dioxide separator 326. Conduit 124 inputs therecycled fuel exhaust into the product side 326B and conduit 334 outputscarbon dioxide depleted exhaust from the product side 326B of carbondioxide separator 326.

Thus, conduits 340 and/or 118 provide an oxidizer to the collection side328A and conduit 330 outputs a humidified oxidizer from the collectionsside 328A of membrane humidifier 328. Conduit 332 inputs carbon dioxideand humid mix into product side 328B and conduit 338 outputs carbondioxide and from the product side 328B.

The humid air or ATO exhaust and carbon dioxide mixture travels from thecollection side of the carbon dioxide separator via carbon dioxideconduit 332 to the membrane humidifier 328. The membrane humidifier 328removes a portion of the water from the humid air mixture, and outputscarbon dioxide and air via output conduit 338. As discussed above, thewater removed from the carbon dioxide conduit 332 by the membranehumidifier 328 may be used to humidify air or ATO exhaust entering themembrane humidifier 328. Thus, system 300 uses ATO exhaust or SOFCcathode exhaust to sweep the carbon dioxide separator collection sideand/or to bias the collection gas with water.

FIG. 4 illustrates a system 400 according to an embodiment of theinvention. The system 400 is similar to system 100 illustrated in FIG. 1and contains a number of components in common. Those components whichare common to both systems 100 and 400 are numbered with the samenumbers in FIGS. 1 and 4 and will not be described further.

One difference between systems 100 and 400 is that system 400 mayutilize a carbon dioxide membrane separator 426 as opposed to a carbondioxide canister trap 126.

A carbon dioxide membrane separator 426 may be a carbon dioxide membraneseparator constructed with tailored membrane structure 429 to blockwater transport from the product side 426B (input side) to thecollection side 426A of the carbon dioxide membrane separator. Thetailored membrane structure, (the product side water block) 429 may beconstructed of a material which allows carbon dioxide to pass, but willnot allow water to pass. One such material which has been found to beeffective for product side water block construction ispolytetrafluoroethylene (Teflon®). The product side water block impedeswater transport via accumulation or blockage into the purging air of thecarbon dioxide membrane separator. The carbon dioxide membrane separator426 may be constructed in a manner similar to an electrochemical carbondioxide separator, but does not require the input of electrical currentto operate.

Recycling conduit 124 may be coupled to the carbon dioxide membraneseparator 426. The recycled fuel exhaust stream enters the product side426B of the carbon dioxide membrane separator 426 via recycling conduit124. The carbon dioxide membrane separator removes carbon dioxide fromthe recycled fuel exhaust stream. As previously discussed, the productside water block 429 of the carbon dioxide membrane separator impedesthe transport of water, so only carbon dioxide is collected by thecarbon dioxide membrane separator 426 on the collection side 426A.Preferably, the carbon dioxide membrane separator 426 removessubstantially all the carbon dioxide from the recycled fuel exhauststream. The carbon dioxide membrane separator 426 may remove less than50% or greater than 50%, such as 50% to 60%, 60% to 70%, 70% to 80%, 80%to 90%, or 90% to 100%, such as about 98%, about 99%, or about 99.5% ofthe carbon dioxide from the recycled fuel exhaust stream.

The purified recycled fuel exhaust stream exiting the collection side426A of the carbon dioxide membrane separator 426 contains less carbondioxide than the recycled fuel exhaust stream that entered the productside 426B of the carbon dioxide separator 426 via the recycling conduit124. As a percentage of overall composition the purified recycled fuelexhaust stream contains a greater percentage of hydrogen than therecycled fuel exhaust stream entering the carbon dioxide separator 426via recycling conduit 124.

The product side 426B of the carbon dioxide membrane separator 426 iscoupled to recycling conduit 434. The purified recycled fuel exhauststream, with a reduced amount of carbon dioxide, is provided back to thefuel inlet stream by the recycling conduit 434. The recycling of reducedcarbon dioxide fuel exhaust into the fuel inlet increases theperformance of the fuel cell stack 106.

Purge air is provided to the collection side 426A of the carbon dioxidemembrane separator 426 via air conduit 430 which is operatively coupledto the collection side 426A of the carbon dioxide membrane separator426. Purge air removes carbon dioxide from the collection side 426A ofthe carbon dioxide membrane separator 426. The carbon dioxide membraneseparator 426 is operatively coupled to output conduit 432 and the airand carbon dioxide mixture flows from the collection side 426A of thecarbon dioxide membrane separator 426 to the output conduit 432.

FIG. 5 illustrates a system 500 according to an embodiment of theinvention. The system 500 is similar to system 100 illustrated in FIG. 1and contains a number of components in common. Those components whichare common to both systems 100 and 500 are numbered with the samenumbers in FIGS. 1 and 5 and will not be described further.

One difference between systems 500 and 100 is that system 500 utilizes awater separator 531 in series with the carbon dioxide canister trap 126.The utilization of the water separator 531 allows water to be removedfrom the portion of the recycled fuel exhaust stream recycled to thefuel cell stack 106. The removal of water from the recycled fuel exhauststream optimizes the steam to carbon ratio and increases cellperformance.

A purified recycled fuel exhaust stream, containing less carbon dioxide,exits the carbon dioxide canister trap 126 via recycling conduit 528 andpasses to a water separator 531. The water separator 531 may be any typewater separator, such as a water condenser separator where steam iscooled to liquid water, which settles to the bottom of the separatorwhile remaining gases (e.g., carbon monoxide, hydrogen, etc) exit viarecycling conduit 534. The water separator 531 continuously removeswater from the purified recycled fuel exhaust stream entering viarecycling conduit 528. A drain in the water separator 531 may providethe collected water to water conduit 533. Preferably the water separator531 removes substantially 50% of the water from the purified recycledfuel exhaust stream. The water separator 530 may remove less than 50%,such as about 50%-40%, 40%-30%, 30%-20%, 20%-10%, 10%-1%, 5%, 0.99%,0.01%, or 0.001% of the water from the purified recycled fuel exhauststream.

The purified recycled fuel exhaust stream exiting the water separator531 contains less water than the purified recycled fuel exhaust streamthat entered the water separator 531 via recycling conduit 528. Comparedto the fuel exhaust stream originally exiting the fuel cell stack 106via fuel exhaust conduit 110, the purified recycled fuel exhaust streamexiting the water separator 531 via recycling conduit 534 contains lesswater and less carbon dioxide overall. The removal of carbon dioxide andwater results in the purified recycled fuel exhaust stream in recyclingconduit 534 having an increased proportion of both hydrogen and carbonmonoxide as a percentage of volume when compared to the fuel exhauststream originally exiting the fuel cell stack 106 via the fuel exhaustconduit 110.

The recycled fuel exhaust stream exits the water separator 531 viarecycling conduit 534 and the purified recycled fuel exhaust stream isprovided back to the fuel inlet stream by the recycling conduit 534. Therecycling of reduced carbon dioxide fuel exhaust into the fuel inletincreases the performance of the fuel cell stack 106 and the reductionof water increases cell performance.

FIG. 6 illustrates a system 600 according to an embodiment of theinvention. The system 600 is similar to system 500 illustrated in FIG. 5and contains a number of components in common. Those components whichare common to both systems 500 and 600 are numbered with the samenumbers in FIGS. 5 and 6 and will not be described further.

One difference between systems 600 and 500 is that system 600 utilizes acombination carbon dioxide canister trap and water separator device 626rather than only a carbon dioxide canister trap 126 and independentwater separator 531. The combination carbon dioxide canister trap andwater separator device 626 may be an integrated carbon dioxide trap andwater separator. The combination carbon dioxide canister trap and waterseparator device 626 functions in a similar manner to produce a purifiedrecycled fuel exhaust stream containing less carbon dioxide and lesswater to recycle to the fuel cell stack 106, the difference being thecarbon dioxide and water are removed at the same time. The combinationcarbon dioxide canister trap and water separator device 626 continuouslyremoves carbon dioxide and water from the recycled fuel exhaust stream.A drain on the combination carbon dioxide canister trap and waterseparator device 626 may provide the collected water to water conduit633. The carbon dioxide canister trap and water separator device 626 mayremove carbon dioxide and water in the same volumes and ratios asdiscussed above in relation to system 500.

The combination carbon dioxide canister trap and water separator device626 receives the recycled fuel exhaust stream via recycling conduit 124.The combination carbon dioxide canister trap and water separator device626 removes carbon dioxide and water from the recycled fuel exhauststream to produce a purified recycled fuel exhaust stream. The purifiedrecycled fuel exhaust stream is passed from the combination carbondioxide canister trap and water separator device 626 to recyclingconduit 634.

The purified recycled fuel exhaust stream exiting the combination carbondioxide canister trap and water separator device 626 contains less waterthan the recycled fuel exhaust stream that entered combination carbondioxide canister trap and water separator device 626 via recyclingconduit 124. Compared to the fuel exhaust stream originally exiting thefuel cell stack 106 via fuel exhaust conduit 110, the purified recycledfuel exhaust stream exiting the combination carbon dioxide canister trapand water separator device 626 via recycling conduit 634 contains lesswater and less carbon dioxide overall. The removal of carbon dioxide andwater results in the purified recycled fuel exhaust stream in recyclingconduit 634 having an increased proportion of both hydrogen and carbonmonoxide as a percentage of volume when compared to the fuel exhauststream originally exiting the fuel cell stack 106 via the fuel exhaustconduit 110.

The purified recycled fuel exhaust stream exits the combination carbondioxide canister trap and water separator device 626 via recyclingconduit 634 and the purified recycled fuel exhaust stream is providedback to the fuel inlet stream by the recycling conduit 634. Therecycling of reduced carbon dioxide fuel exhaust into the fuel inletincreases the performance of the fuel cell stack 106 and the reductionof water increases cell performance.

FIG. 7 illustrates a system 700 according to an embodiment of theinvention. The system 700 is similar to system 300 illustrated in FIG. 3and contains a number of components in common. Those components whichare common to both systems 300 and 700 are numbered with the samenumbers in FIGS. 3 and 7 and will not be described further.

One difference between systems 300 and 700 is that system 700 utilizes awater separator 731 in series with the carbon dioxide separator 326. Theutilization of the water separator 731 allows water to be removed fromthe recycled fuel exhaust stream recycled to the fuel cell stack 106.The removal of water from the recycled fuel exhaust stream increasescell performance.

A purified recycled fuel exhaust stream, containing less carbon dioxide,exits the carbon dioxide separator 326 via recycling conduit 728 andpasses to a water separator 730. The water separator 731 may be any typewater separator, such as a water condenser separator. The waterseparator 731 continuously removes water from the purified recycled fuelexhaust stream entering via recycling conduit 728.

A drain on the water separator 731 may provide the collected water towater conduit 733. Water conduit 733 is operatively coupled to theproduct side 328B of the membrane humidifier 328, and provides water tothe membrane humidifier 328. The presence of water received from thewater separator 731 via the water conduit 733 may eliminate the need forwater conduit 342 present in system 300.

The purified recycled fuel exhaust stream exiting the water separator731 contains less water than the purified recycled fuel exhaust streamthat entered the water separator 731 via recycling conduit 728. Comparedto the fuel exhaust stream originally exiting the fuel cell stack 106via fuel exhaust conduit 110, the purified recycled fuel exhaust streamexiting the water separator 731 via recycling conduit 734 contains lesswater and less carbon dioxide overall. The removal of carbon dioxide andwater results in the purified recycled fuel exhaust stream in recyclingconduit 734 having an increased proportion of both hydrogen and carbonmonoxide as a percentage of volume when compared to the fuel exhauststream originally exiting the fuel cell stack 106 via the fuel exhaustconduit 110.

The purified recycled fuel exhaust stream exits the water separator 731via recycling conduit 734 and the purified recycled fuel exhaust streamis provided back to the fuel inlet stream by the recycling conduit 734.The recycling of reduced carbon dioxide fuel exhaust into the fuel inletincreases the performance of the fuel cell stack 106 and the reductionof water optimizes the steam to carbon ratio and increases cellperformance.

FIG. 8 illustrates a system 800 according to an embodiment of theinvention. The system 800 is similar to system 700 illustrated in FIG. 7and contains a number of components in common. Those components whichare common to both systems 700 and 800 are numbered with the samenumbers in FIGS. 7 and 8 and will not be described further.

One difference between systems 800 and 700 is that system 800 utilizes acombination carbon dioxide and water separator 826 rather than only acarbon dioxide separator 326 and independent water separator 731. Thecombination carbon dioxide and water separator 826 functions in asimilar manner to produce a purified recycled fuel exhaust streamcontaining less carbon dioxide and less water to recycle to the fuelcell stack 106, the difference being the carbon dioxide and water areremoved at the same time. The combination carbon dioxide and waterseparator 826 continuously removes carbon dioxide and water from therecycled fuel exhaust stream.

The combination carbon dioxide and water separator 826 receives therecycled fuel exhaust stream via recycling conduit 124. The combinationcarbon dioxide and water separator 826 removes carbon dioxide and waterfrom the recycled fuel exhaust stream to produce a purified recycledfuel exhaust stream. The purified recycled fuel exhaust stream is passedfrom combination carbon dioxide and water separator 826 to recyclingconduit 834.

The purified recycled fuel exhaust stream exiting the combination carbondioxide and water separator 826 contains less water than the recycledfuel exhaust stream that entered the combination carbon dioxide andwater separator 826 via recycling conduit 124. Compared to the fuelexhaust stream originally exiting the fuel cell stack 106 via fuelexhaust conduit 110, the purified recycled fuel exhaust stream exitingthe combination carbon dioxide and water separator 826 via recyclingconduit 834 contains less water and less carbon dioxide overall. Theremoval of carbon dioxide and water results in the purified recycledfuel exhaust stream in recycling conduit 828 having an increasedproportion of both hydrogen and carbon monoxide as a percentage ofvolume when compared to the fuel exhaust stream originally exiting thefuel cell stack 106 via the fuel exhaust conduit 110.

The purified recycled fuel exhaust stream exits the combination carbondioxide and water separator 826 via recycling conduit 834 and thepurified recycled fuel exhaust stream is provided back to the fuel inletstream by the recycling conduit 828. The recycling of reduced carbondioxide fuel exhaust into the fuel inlet increases the performance ofthe fuel cell stack 106 and the reduction of water optimizes the steamto carbon ratio and increases cell performance.

The water and carbon dioxide removed from the combination carbon dioxideand water separator 826 mixes with purge air received from humid airconduit 330 and exits the combination carbon dioxide and water separator826 via carbon dioxide conduit 832. The membrane humidifier 328 removeswater from the carbon dioxide, water, and air mixture received viacarbon dioxide conduit 832. The water removed by the membrane humidifier328 may be used to humidify the input air to the membrane humidifier328. In this manner, the need for water conduit 342 present in system300 may be eliminated.

FIG. 9 illustrates a system 900 according to an embodiment of theinvention. The system 900 is similar to system 400 illustrated in FIG. 4and contains a number of components in common. Those components whichare common to both systems 400 and 900 are numbered with the samenumbers in FIGS. 4 and 9 and will not be described further.

One difference between systems 400 and 900 is that system 900 utilizes awater separator 931 in series with the carbon dioxide membrane separator426. The utilization of the water separator 931 allows water to beremoved from the portion of the recycled fuel exhaust stream recycled tothe fuel cell stack 106. The removal of water from the recycled fuelexhaust stream increases cell performance.

A purified (e.g., carbon dioxide depleted) recycled fuel exhaust streamexits the carbon dioxide membrane separator 426 via recycling conduit928 and passes to the water separator 931. The water separator 931 maybe any type water separator, such as a water condenser separator. Thewater separator 931 continuously removes water from the recycled fuelexhaust stream entering via recycling conduit 928. A drain on the waterseparator 936 may provide the collected water to water conduit 933.

The purified recycled fuel exhaust stream exiting the water separator931 contains less water than the purified recycled fuel exhaust streamthat entered the water separator 931 via recycling conduit 928. Comparedto the fuel exhaust stream originally exiting the fuel cell stack 106via fuel exhaust conduit 110, the purified recycled fuel exhaust streamexiting the water separator 931 via recycling conduit 934 contains lesswater and less carbon dioxide overall. The removal of carbon dioxide andwater results in the purified recycled fuel exhaust stream in recyclingconduit 934 having an increased proportion of both hydrogen and carbonmonoxide as a percentage of volume when compared to the fuel exhauststream originally exiting the fuel cell stack 106 via the fuel exhaustconduit 110.

The purified recycled fuel exhaust stream exits the water separator 931via recycling conduit 934 and the purified recycled fuel exhaust streamis provided back to the fuel inlet stream by the recycling conduit 934.The recycling of reduced carbon dioxide fuel exhaust into the fuel inletincreases the performance of the fuel cell stack 106 and the reductionof water optimizes the steam to carbon ratio and increases cellperformance.

In an alternative embodiment, (not shown), a carbon dioxide membraneseparator 926 described in connection with system 900 may be combinedwith a water separator 936 in the same housing. In this manner theseparation of carbon dioxide would occur in the same housing as theseparation of water, but the carbon dioxide membrane separator 926 andwater separator 936 would remain separate apparatuses.

FIG. 10 illustrates a system 1000 according to an embodiment of theinvention. The system 1000 is similar to system 300 illustrated in FIG.3 and contains a number of components in common. Those components whichare common to both systems 300 and 1000 are numbered with the samenumbers in FIGS. 3 and 10 and will not be described further.

One difference between systems 300 and 1000 is that system 1000 utilizesspray humidifier 1028 to bias carbon dioxide separator 326 by addingwater to the collection side 326A of the carbon dioxide separator 326,rather than the membrane humidifier 328 of system 300. Additionally, insystem 1000 the air conduit 118 need not be connected to the sprayhumidifier 1042.

The spray humidifier 1028 is used to add water to the air that will beinput to the collection side 326A of the carbon dioxide separator 326.

Air is input to the spray humidifier 1028 via air conduit 1038. Watermay be input to the spray humidifier 1028 via a water conduit 1042.Water may also be input to the spray humidifier via water conduit 1033.Water is sprayed into the air input into the spray humidifier 1028 andmixes with the air to produce humid air. The now humid air passes tohumid air conduit 330.

Humid air conduit 330 is coupled to the collection side 326A of thecarbon dioxide separator 326 and the humid air is used to bias theseparation of carbon dioxide by the carbon dioxide separator 326. Wherea traditional carbon dioxide separator naturally selects for water in areaction, the presence of water on the collection side of the carbonseparator reduces the selection of water and increases the efficiency ofthe carbon dioxide separator to select for carbon dioxide. In thismanner the increase amount of water in the air entering the carbondioxide separator 326 biases the collection side 326A of the carbondioxide separator 326 to select for carbon dioxide from the recycledfuel exhaust stream.

The humid air and carbon dioxide mixture travels from the carbon dioxideseparator via carbon dioxide conduit 1032 to a condenser 1031. Thecondenser 1031 removes a portion of the water from the humid air andcarbon dioxide mixture, and outputs carbon dioxide and air via outputconduit 1036. The water collected in the condenser 1031 may be providedto water conduit 1033 and input to the spray humidifier 1028.

FIG. 11 illustrates a system 1100 according to an embodiment of theinvention. The system 1100 is similar to system 700 illustrated in FIG.7 and contains a number of components in common. Those components whichare common to both systems 700 and 1100 are numbered with the samenumbers in FIGS. 7 and 11 and will not be described further.

One difference between systems 700 and 1100 is that system 1100 utilizesspray humidifier 1028 to bias carbon dioxide separator 326 by addingwater to the collection side of the carbon dioxide separator 326, ratherthan the membrane humidifier 328 of system 700. Additionally, in system1100 the air conduit 118 need not be connected to the spray humidifier1042 and contains two water separators 731 and 1031. Water separator 731is located at the output of the product side 326B of the carbon dioxideseparator 326 and water separator 1031 is located at the output of thecollection side 326A of the carbon dioxide separator 326. Thus, system1100 is a combination of systems 700 and 1000.

A drain on the water separator 730 may provide the collected water towater conduit 733. Water conduit 733 is operatively coupled to the sprayhumidifier 1028, and provides water to the spray humidifier 1028.

The spray humidifier 1028 is used to add water to the air that will beinput to the collection side of the carbon dioxide separator 326.

Air is input to the spray humidifier 1028 via air conduit 1038. Watermay be input to the spray humidifier 1028 from the water separator 731via water conduit 733. Water may also be input to the spray humidifierfrom condenser 1031 via water conduit 1033. Water is sprayed into theair input into the spray humidifier 1028 and mixes with the air toproduce humid air. The now humid air passes to humid air conduit 330.

Humid air conduit 330 is coupled to the collection side 326A of thecarbon dioxide separator 326 and the humid air is used to bias theseparation of carbon dioxide by the carbon dioxide separator 326. Wherea traditional carbon dioxide separator naturally selects for water in areaction, the presence of water on the collection side of the carbonseparator reduces the selection of water and increases the efficiency ofthe carbon dioxide separator to select for carbon dioxide. In thismanner the increase amount of water in the air entering the collectionside 326A of the carbon dioxide separator 326 biases the carbon dioxideseparator 326 to select for carbon dioxide from the recycled fuelexhaust stream.

The humid air and carbon dioxide mixture travels from the carbon dioxideseparator via carbon dioxide conduit 1032 to a water separator, such asa condenser 1031. The condenser 1031 removes a portion of the water fromthe humid air and carbon dioxide mixture, and outputs carbon dioxide andair via output conduit 1036. The water collected in the condenser 1031may be provided to water conduit 1033 and input to the spray humidifier1028.

FIG. 12 illustrates a system 1200 according to an embodiment of theinvention. The system 1200 is similar to system 800 illustrated in FIG.8 and contains a number of components in common. Those components whichare common to both systems 800 and 1200 are numbered with the samenumbers in FIGS. 8 and 12 and will not be described further.

One difference between systems 800 and 1200 is that system 1200 utilizesspray humidifier 1028 to bias the combination carbon dioxide and waterseparator 826 by adding water to the collection side of the combinationcarbon dioxide and water separator 826, rather than utilizing themembrane humidifier 328 of system 800. Additionally, in system 1200 theair conduit 118 need not be connected to the spray humidifier 1028.System 1200 is a combination of systems 800 and 1000 in that it containsthe combination carbon dioxide and water separator 826 and the sprayhumidifier 1028.

The spray humidifier 1028 is used to add water to the air that will beinput to the collection side of the carbon dioxide separator 826.

The water and carbon dioxide removed from the combination carbon dioxideand water separator 826 mixes with purge air received from humid airconduit 330 and exits the combination carbon dioxide and water separator826 via carbon dioxide conduit 1232.

Air is input to the spray humidifier 1028 via air conduit 1038. Watermay be input to the spray humidifier 1028 via a water conduit 1233.Water is sprayed into the air input into the spray humidifier 1028 andmixes with the air to produce humid air. The now humid air passes tohumid air conduit 330 to be provided to the collection side of thecarbon dioxide and water separator 826.

The humid air and carbon dioxide mixture travels from the combinationcarbon dioxide and water separator 826 via carbon dioxide conduit 1232to a water separator, such as the condenser 1231. The condenser 1231removes a portion of the water from the humid air and carbon dioxidemixture, and outputs carbon dioxide and air via output conduit 1236. Thewater collected in the condenser 1231 may be provided to water conduit1233 and input to the spray humidifier 1028.

FIG. 13 illustrates a system 1300 according to an embodiment of theinvention. The system 1300 is similar to system 100 illustrated in FIG.1 and contains a number of components in common. Those components whichare common to both systems 100 and 1300 are numbered with the samenumbers in FIGS. 1 and 13 and will not be described further.

One difference between systems 100 and 1300 is that system 1300 mayutilize a water separator 1301 as opposed to a carbon dioxide canistertrap 126 or another carbon dioxide separator. The utilization of thewater separator 1301 allows water to be removed from the portion of therecycled fuel exhaust stream recycled to the fuel cell stack 106. Insystem 1300, the recirculation blower 122 is coupled to recyclingconduit 120 to provide the recycled fuel exhaust stream from recyclingconduit 120 to a water separator 1301 via recycling conduit 124. Thus,in this embodiment, water rather than carbon dioxide is removed from thefuel exhaust stream.

The water separator 1301 may be any type water separator, such as an aircooled water condenser separator where steam is cooled to liquid water,which settles to the bottom of the separator by gravity, while remaininggases (e.g., carbon monoxide, carbon dioxide, hydrogen, etc) exit viarecycling conduit 1305. The water separator 1301 continuously removeswater from the recycled fuel exhaust stream entering via recyclingconduit 124. A drain in the water separator 1301 may provide thecollected water to water conduit 1303. Water conduit 1303 may dischargethe collected water away from the fuel inlet stream or additionally outof the system 1300. Preferably the water separator 1301 removes up to65% of the water from the recycled fuel exhaust stream. The waterseparator 1301 may remove less than 65%, such as about 1-50%, 50%-40%,including 40%-30%, 30%-20%, 20%-10%, or 10%-1% of the water from therecycled fuel exhaust stream.

The recycled fuel exhaust stream exiting the water separator 1301contains less water than the recycled fuel exhaust stream that enteredthe water separator 1301 via recycling conduit 124. Compared to the fuelexhaust stream originally exiting the fuel cell stack 106 via fuelexhaust conduit 110, the recycled fuel exhaust stream exiting the waterseparator 1301 via recycling conduit 1305 contains less water overall(i.e., is a drier recycled fuel exhaust stream). The removal of waterresults in the drier recycled fuel exhaust stream in recycling conduit1305 having an increased proportion of hydrogen, carbon monoxide, andcarbon dioxide as a percentage of volume when compared to the fuelexhaust stream originally exiting the fuel cell stack 106 via the fuelexhaust conduit 110.

The drier recycled fuel exhaust stream exits the water separator 1301via recycling conduit 1305 and the drier recycled fuel exhaust stream isprovided back to the fuel inlet stream by the recycling conduit 1305.The removal of water from the recycled fuel exhaust stream optimizes thesteam to carbon ratio and increases cell performance, and the recyclingof fuel exhaust containing less water into the fuel inlet increases theperformance of the fuel cell stack 106. SOFC fuel cells using naturalgas fuel produce chemical byproducts of two thirds water and one thirdcarbon dioxide. These byproducts dilute the fuel to the point that it isnot practical to function with a fuel utilization of greater thanapproximately 88% including partial recirculation of the anode exhaust.Separation and removal of a portion of the water in the recirculatedanode exhaust allows an increase in fuel utilization to over 89%, suchas 90-95%, such as about 95% and a resultant 3 to 4 efficiency pointsincrease. Fuel cells generally degrade in performance over time andproduce more waste heat in the process. This results in the need tocombust less or no fuel for balancing the heat loss and removal of waterand/or carbon dioxide allows a higher fuel utilization within the activefuel cells. Since both the product water and the product carbon dioxidecause the same negative Nernst voltage effect, the inventors realizedthat water, being twice the volume of carbon dioxide, would have abigger impact in its removal than removing carbon dioxide. Waterseparated from the fuel (anode) recirculation loop can allow fuelutilization to be increased into the mid 90's percentile, e.g., such as90-95%, and efficiency gains up to about 4 points, but not exceedingabout 64% based on the fuel LHV. Therefore, the water separator in theanode recycle loop achieves up to about 95% fuel utilization byincreasing the recycle rate from 58% up to about 85% while removing upto about 65% of the product water when the SOFC system requires lessfuel combustion for heat balance. This is accomplished while maintainingthe single pass fuel utilization at about 75% and the fuel inlet oxygento carbon ratio at 2.0.

In an alternative embodiment, all or a portion of the drier recycledfuel exhaust stream may be passed to a hydrogen separator 1311. Thehydrogen separator 1311 is optional and is preferably omitted. A valve1307 may direct all or a portion of the drier recycled fuel exhauststream from recycling conduit 1305 into conduit 1309 and valve 1307 maydirect all or a portion of the drier recycled fuel exhaust stream fromrecycling conduit 1305 to conduit 1308. Conduit 1309 may be coupled toair exhaust conduit 112 and may provide the drier recycled fuel exhauststream to air exhaust conduit 112 via which the drier recycled fuelexhaust stream may be provided to the ATO reactor 116.

Conduit 1308 may be coupled to a hydrogen separator 1311. Hydrogenseparator 1311 may be any type hydrogen separator, such as a cascadedelectrochemical hydrogen pump separation unit which electrochemicallyseparates hydrogen from the drier recycled fuel exhaust stream. Thehydrogen separator 1311 may separate about 95%, such as 95% to about100% of the hydrogen contained in the drier recycled fuel exhaust streamentering via conduit 1308. The separated hydrogen may be provided to thefuel inlet stream by hydrogen conduit 1313. The remaining gases in thedrier recycled fuel exhaust stream may exit the hydrogen separator 1311via conduit 1314 which may be coupled to conduit 1309. In this mannerthe remaining gases in the drier recycled fuel exhaust stream may beprovided to the ATO reactor 116.

FIG. 14 illustrates a system 1400 according to an embodiment of theinvention. The system 1400 is a specific embodiment of system 1300illustrated in FIG. 13 and contains a number of components in common.Those components which are common to both systems 1300 and 1400 arenumbered with the same numbers in FIGS. 13 and 14 and will not bedescribed further.

System 1400 is a specific embodiment of system 1300 in which a watermembrane separator 1402 biased by air is used. The air flow controlledselective water vapor membrane separator 1402 preferably removes excesswater to maintain the fuel inlet oxygen to carbon ration at 2.0 and maydischarge the water vapor into the atmosphere. The utilization of thewater membrane separator 1402 biased by air allows water to be removedfrom the portion of the recycled fuel exhaust stream recycled to thefuel cell stack 106. The water membrane separator 1402 continuouslyremoves water from the recycled fuel exhaust stream entering viarecycling conduit 124.

Water membrane separator 1402 may comprise a polymeric membraneseparator. The membrane 1404 of water membrane separator 1402 may be aNafion® membrane. For example, one separator made by Perma Pure, LLC isbased on Nafion® membrane tubes within a 316 stainless steel housing.The Nafion® is specified by its manufacturer DuPont to operate up to190C. When used with gasses as a dryer, it is specified by Perma Pure tooperate at temperatures up to 150C. A pair of one foot long drying unitsat about 2.5 inches in diameter operating in a vertical parallelarrangement has acceptable pressure drop and drying capability. Thewater membrane separator 1402 may be oriented in any direction. In apreferred embodiment the water membrane separator 1402 may be avertically positioned tubular membrane separator (i.e., positioned suchthat the central axis of the membrane tube is vertical in relation tothe ground). The selective water vapor membrane separator embodiment hasthe advantages of being low cost, low parasitic power, easy tointegrate, compatible with carbon dioxide sequestration, and dischargesthe water vapor into the atmosphere. Water may be collected by the watermembrane separator 1402 from recycling conduit 124 on the product side1402B of the water membrane separator 1402. Water permeates across themembrane 1404 from the product side 1402B to collection side 1402A ofwater membrane separator 1402. The partial pressure of the water in theproduct side 1402B of the water membrane separator 1402 drives diffusionof the water across the membrane 1404 to the collection side 1402A ofthe water membrane separator 1402. Preferably the water membraneseparator 1402 removes substantially 50% of the water from the recycledfuel exhaust stream. The water membrane separator 1402 may remove up to65%, such as about 1-50%, 50%-40%, 40%-30%, 30%-20%, 20%-10%, or 10%-1%of the water from the recycled fuel exhaust stream.

The drier recycled fuel exhaust stream exiting the water membraneseparator 1402 contains less water than the recycled fuel exhaust streamthat entered the water membrane separator 1402 via recycling conduit124. Compared to the fuel exhaust stream originally exiting the fuelcell stack 106 via fuel exhaust conduit 110, the drier recycled fuelexhaust stream exiting the water membrane separator 1402 via recyclingconduit 1305 contains less water overall. The removal of water resultsin the drier recycled fuel exhaust stream in recycling conduit 1305having an increased proportion of hydrogen, carbon monoxide, and carbondioxide as a percentage of volume when compared to the fuel exhauststream originally exiting the fuel cell stack 106 via the fuel exhaustconduit 110.

The drier recycled fuel exhaust stream exits the water membraneseparator 1402 from the product side 1402B via recycling conduit 1305and the drier recycled fuel exhaust stream is provided back to the fuelinlet stream by the recycling conduit 1305.

Air may be provided to the collection side 1402A of the water membraneseparator 1402 via air conduit 1408 which is operatively coupled to thecollection side 1402A of the water membrane separator 1402. An airblower (not shown) may be used to blow air into conduit 1408. The airremoves water from the collection side 1402A of the water membraneseparator 1402. The water membrane separator 1402 is operatively coupledto discharge conduit 1410 and the air and water mixture flows from thecollection side 1402A of the water membrane separator 1402 to thedischarge conduit 1410. Discharge conduit 1410 may discharge the air andevaporated water mixture away from the fuel inlet stream or additionallyout of the system 1400, for example into the atmosphere as humid air orwater vapor and air. The addition of air to the collection side 1402A ofthe water membrane separator 1402 biases the water membrane separator1402 such that the partial pressure of water on the collection side1402A is less than the partial pressure of water on the product side1402B. The difference in partial pressure drives the diffusion of wateracross the membrane 1404 of the water membrane separator 1402.

FIGS. 13 and 14 illustrate embodiments of fuel cell systems in which allthe fuel exhaust is recycled into a water separator before any fuelexhaust is provided into the fuel inlet stream. The overall advantagesof the embodiments of FIGS. 13 and 14 are to increase the overallaverage efficiency of the SOFC system by up to 4 percentage points andextending the SOFC system lifetime at any specific voltage level. Bothembodiments have the advantage of being able to use commerciallyavailable components to fashion the system. The specific advantages ofeach embodiment are: the water condenser separator 1302 embodiment ofFIG. 13 has the advantages of being low cost, low parasitic power, easyto integrate, and compatible with carbon dioxide sequestration; and theselective water vapor membrane separator 1402 embodiment of FIG. 14 hasthe advantages of being low cost, low parasitic power, easy tointegrate, compatible with carbon dioxide sequestration, and dischargesthe water vapor into the atmosphere.

FIG. 15 illustrates a system 1500 according to an embodiment of theinvention. The system 1500 is similar to system 400 illustrated in FIG.4 and contains a number of components in common. Those components whichare common to both systems 400 and 1500 are numbered with the samenumbers in FIGS. 4 and 15 and will not be described further.

One difference between systems 400 and 1500 is that system 1500 utilizesa vacuum pump 1504 to remove separated carbon dioxide from thecollection side 426A of the carbon dioxide membrane separator 426 ratherthan purge air. The utilization of a vacuum pump 1504 may be moreeffective than purge air, and the parasitic power draw of the vacuumpump 1504 may not be so large as to overcome the benefit of using thevacuum pump 1504. An output conduit 1502 may be operatively connected tothe collection side 426A of the carbon dioxide membrane separator 426.The output conduit 1502 may be operatively connected to the vacuum pump1504. An output conduit 1506 may be coupled to the vacuum pump 1504. Inoperation, the vacuum pump 1504 may pull carbon dioxide from thecollection side 426A of the carbon dioxide membrane separator 426 viaoutput conduit 1502.

In an alternative embodiment (not shown) nitrogen rather than air may beused as the purge gas for carbon dioxide separators. In anotheralternative embodiment (not shown) the membrane of a carbon dioxideseparator may include amine.

The fuel cell systems described herein may have other embodiments andconfigurations, as desired. Other components, such as fuel side exhauststream condensers, heat exchangers, heat-driven pumps, turbines,additional gas separation devices, hydrogen separators which separatehydrogen from the fuel exhaust and provide hydrogen for external use,fuel processing subsystems, fuel reformers and or water gas shiftreactors, may be added if desired. Furthermore, it should be understoodthat any system element or method steps described in any embodimentand/or illustrated in any figure may also be used in systems and/ormethods of other suitable embodiments described above even if such useis not expressly described.

The foregoing description of the invention has been presented forpurposes of illustration and description. It is not intended to beexhaustive or to limit the invention to the precise form disclosed, andmodifications and variations are possible in light of the aboveteachings or maybe acquired a practice of the invention. The descriptionwas chosen in order to explain the principles of the invention and itspractical application. It is intended that the scope of the invention asdefined by the claims appended hereto, and their equivalents.

1. A fuel cell system, comprising: a fuel cell stack; a first recyclingconduit operatively connecting the fuel cell stack to a carbon dioxideseparation device, the first recycling conduit adapted to recycle aportion of a fuel exhaust stream from the fuel cell stack to the carbondioxide separation device, the carbon separation device adapted toremove carbon dioxide from the recycled fuel exhaust stream creating apurified recycled fuel exhaust stream; a second recycling conduitoperatively connecting the carbon dioxide separation device to the fuelinlet conduit, the second recycling conduit adapted to provide thepurified recycled fuel exhaust stream to the fuel inlet conduit; and anexhaust conduit operatively connecting the fuel cell stack to acollection side of the carbon dioxide separation device, the exhaustconduit adapted to provide a second portion of the fuel exhaust streamfrom the fuel cell stack to sweep carbon dioxide from the collectionside of the carbon dioxide separation device.
 2. The system of claim 1,further comprising: an air conduit operatively connected to the exhaustconduit, the air conduit adapted to provide air into the second portionof the fuel exhaust stream provided to the collection side of the carbondioxide separation device.
 3. The system of claim 1, further comprising:a membrane humidifier operatively connected to the exhaust conduit, themembrane humidifier adapted to provide water into the second portion ofthe fuel exhaust stream provided to the collection side of the carbondioxide separation device.
 4. The system of claim 1, further comprising:a water separator operatively connected to the second recycling conduit,the water separator adapted to remove water from the purified recycledfuel exhaust stream; a membrane humidifier operatively connected to theexhaust conduit, the membrane humidifier adapted to provide water intothe second portion of the fuel exhaust stream provided to the collectionside of the carbon dioxide separation device; and a water conduitoperatively connected to the water separator and membrane humidifier,the water conduit adapted to provide water from the water separator tothe membrane humidifier.
 5. The system of claim 1, wherein the carbondioxide separation device is a combination carbon dioxide and waterseparation device further adapted to remove water from the recycled fuelexhaust stream, and the fuel cell system further comprising: a membranehumidifier operatively connected to the exhaust conduit, the membranehumidifier adapted to provide water into the second portion of the fuelexhaust stream provided to the collection side of the carbon dioxideseparation device; and a water conduit operatively connected to thecarbon dioxide separation device, the water conduit adapted to providewater from the carbon dioxide separation device to the membranehumidifier.
 6. The system of claim 1, wherein the fuel cell stack is asolid oxide fuel cell (SOFC) stack, the fuel cell system furthercomprising: an anode tail gas oxidizer operatively connected to theexhaust conduit, the anode tail gas oxidizer adapted to oxidize SOFCanode and cathode exhaust prior to providing the second portion of thefuel exhaust stream from the fuel cell stack to the collection side ofthe carbon dioxide separation device.
 7. A fuel cell system, comprising:a fuel cell stack; a first recycling conduit operatively connecting thefuel cell stack to a carbon dioxide separation device, the firstrecycling conduit adapted to recycle a portion of a fuel exhaust streamfrom the fuel cell stack to the carbon dioxide separation device, thecarbon separation device adapted to remove carbon dioxide from therecycled fuel exhaust stream creating a purified recycled fuel exhauststream; a second recycling conduit operatively connecting the carbondioxide separation device to the fuel inlet conduit, the secondrecycling conduit adapted to provide the purified recycled fuel exhauststream to the fuel inlet conduit; a humidifier, adapted to humidify air;and an air conduit operatively coupled to the humidifier and the carbondioxide separation device, the air conduit adapted to provide humid airto a collection side of the carbon dioxide separation device.
 8. Thesystem of claim 7, further comprising: a water separator operativelyconnected to the second recycling conduit, the water separator adaptedto remove water from the purified recycled fuel exhaust stream; and awater conduit operatively connected to the water separator and thehumidifier, the water conduit adapted to provide water from the waterseparator to the humidifier.
 9. The system of claim 7, wherein thecarbon dioxide separation device is a combination carbon dioxide andwater separation device further adapted to remove water from therecycled fuel exhaust stream, and the fuel cell system furthercomprising: a water conduit operatively connected to the carbon dioxideseparation device, the water conduit adapted to provide water from thecarbon dioxide separation device to the humidifier.
 10. The system ofclaim 7, wherein the humidifier is a spray humidifier.
 11. The system ofclaim 7, wherein the first recycling conduit is operatively connected toa product side of the carbon separation device and adapted to providethe fuel exhaust stream to the product side of the carbon separationdevice, wherein the second recycling conduit is operatively connected tothe product side of the carbon separation and adapted to receivepurified recycled fuel exhaust from the product side of the carbonseparation device, wherein the carbon dioxide separation device isadapted to enable carbon dioxide to diffuse through a separator from theproduct side to the collection side of the carbon dioxide separationdevice, and wherein the fuel cell stack is a solid oxide fuel cell(SOFC) stack, the system further comprising: a reactor operativelyconnected to the fuel cell stack, the reactor adapted to oxidize a SOFCfuel exhaust stream using a SOFC air exhaust stream to generate areactor exhaust stream; and an exhaust conduit operatively connected tothe reactor and the humidifier, the exhaust conduit adapted to providethe reactor exhaust stream to the humidifier.
 12. The system of claim10, wherein the carbon dioxide separation device is an electrochemicalseparator or a membrane separator.
 13. A fuel cell system, comprising: afuel cell stack; a first recycling conduit operatively connecting thefuel cell stack to a carbon dioxide membrane separator, the firstrecycling conduit adapted to recycle a portion of a fuel exhaust streamfrom the fuel cell stack to the carbon dioxide membrane separator, thecarbon dioxide membrane comprising a carbon separating membranecontaining a tailored membrane adapted to selectively transport morecarbon dioxide than water from the recycled fuel exhaust, the carbondioxide membrane separator thereby adapted to remove carbon dioxide fromthe recycled fuel exhaust stream creating a purified recycled fuelexhaust stream; and a second recycling conduit operatively connectingthe carbon dioxide membrane separator to the fuel inlet conduit, thesecond recycling conduit adapted to provide the purified recycled fuelexhaust stream to the fuel inlet conduit.
 14. The system of claim 13,wherein the tailored membrane comprises a polytetrafluoroethylenemembrane.
 15. The system of claim 13, further comprising a waterseparator operatively connected to the second recycling conduit, thewater separator adapted to remove water from the purified recycled fuelexhaust stream.
 16. The system of claim 13, further comprising an airconduit operatively connected to a collection side of the carbon dioxidemembrane separator, the air conduit adapted to provide purge air to thecollection side of the carbon dioxide membrane separator.
 17. The systemof claim 13, further comprising: an output conduit operatively connectedto a collection side of the carbon dioxide membrane separator; and avacuum pump operatively connected to the output conduit, the vacuum pumpadapted to pull carbon dioxide from the collection side of the carbondioxide membrane separator via the output conduit.
 18. A fuel cellsystem, comprising: a fuel cell stack; a first recycling conduitoperatively connecting the fuel cell stack to a water separator, thefirst recycling conduit adapted to recycle a portion of a fuel exhauststream from the fuel cell stack to the water separator, the waterseparator adapted to remove water from the recycled fuel exhaust streamcreating a drier recycled fuel exhaust stream before any portion of thefuel exhaust stream is recycled into the fuel inlet stream; a secondrecycling conduit operatively connecting the water separator to the fuelinlet conduit, the second recycling conduit adapted to provide the drierrecycled fuel exhaust stream to the fuel inlet conduit; and a dischargeconduit operatively connected to the water separator and adapted todischarge the removed water out of the water separator and away from thefuel inlet conduit.
 19. The system of claim 18, wherein the waterseparator is a vertically positioned water membrane separator, whereinthe first recycling conduit is operatively connected to a product sideof the water membrane separator and adapted to provide the fuel exhauststream to the product side of the water membrane separator, wherein thesecond recycling conduit is operatively connected to the product side ofthe water membrane separator and adapted to receive the drier recycledfuel exhaust from the product side of the water membrane separator, andwherein the water membrane separator is adapted to enable water todiffuse through a membrane from the product side to a collection side ofthe water membrane separator, the system further comprising: an airconduit operatively connected to the collection side of the watermembrane separator, the air conduit adapted to provide air into thecollection side of the water membrane separator; and wherein thedischarge conduit is operatively connected to the collection side of thewater separator and adapted to discharge the removed water and air outof the water membrane separator and away from the fuel inlet conduit.